LAG: Accept user specified lag.ifindex, also when listed in descending order

[jbemmel]

Fixes #2246

Tested: ./device-module-test -d eos -p clab lag

[ipspace]

This one looks a bit fishy. Have to check what happens if you have three LAGs and you number then "2, 1, none".

Will merge after 2.0.0 goes out.

[jbemmel]

This one looks a bit fishy. Have to check what happens if you have three LAGs and you number then "2, 1, none".

That would assign overlapping ifindex values. The code is not currently trying to be overly sophisticated about auto-assigning missing ifindex values - it would mean keeping track of a set per node. Could be added

We could also change it such that the user has to specify all ifindex values if they decide to do one. I manually assign values to model what's configured, but in general I'd expect most users to leave them auto-assigned